It is now well recognized that hypothalamic and pituitary hormones are secreted in a pulsatile pattern which is unique for each hormone and which may vary according to the physiological status of the subject. The evidence we have obtained supports the concept that the pulsatile secretory pattern contains encoded messages that convey the required inputs to elicit secretory responses and other important biological events, such as cell differentiation and even enhanced gene expression. It seems evident, therefore, that pulsatile hormone secretion represents a sophisticated, carefully regulated means of intracellular communication. We have evaluated the characteristics of the pulsatile pattern of secretion of most pituitary hormones and of some hypothalamic peptides as well. These studies indicate that several parameters of the pulsatile pattern can change during different physiological situations or after specific pharmacological interventions. Secretion of the neuropeptide LHRH into the hypophysial portal blood in intact animals occurs in a pulsatile fashion. Evaluation of the total amount (mass) of hormone secreted in each pulse (measuring area under the pulse) reveals that at least two distinct populations of pulses can be separated, i.e., "small" and "big" mass pulses. Orchidectomy results in an almost complete disappearance of "big mass" pulses. Testosterone replacement reestablishes the presence of large mass pulses. These observations are helping to re-define the established dogma of negative steroid feedback, into a new concept in which the steroids interact with neural structures to modify the pulse pattern of peptide release. This may be accomplished by establishing a functional neuronal network capable of generating a pulsatile pattern of LHRH secretion which can appropriately maintain pituitary-gonadal function. Additional studies on the pulsatile pattern of hormones under dual (stimulatory/inhibitory) control (such as prolactin) or under multifactorial neural regulation (ACTH) also provided very useful information about the encoding of signals on the pulsatile pattern which may contribute to the pleiotropic actions of these hormones.